Types of Patellar Instability and Treatment Guidelines
Shital N. Parikh
Marios G. Lykissas
INTRODUCTION
As knee extension became an important feature of bipedalism to increase stride length and walking efficiency, the patella became free from its trochlear constraints. The unfortunate effect of this evolutionary feature of hominid patellofemoral joint has been patellar instability, which has not been seen in other primates, like chimpanzees, which keep their knees flexed at all times.1
Another epigenetic feature of hominids has been the development of femoral obliquity or genu valgum to help facilitate knee flexion-extension in a parasagittal plane and help position the knee closer to the body’s center of gravity during stride. This has led to an increase in the lateral vector and force acting on the patella that might displace the patellar laterally during quadriceps contraction. Such femoral obliquity is not seen in nonhuman primates.1
The patella is the largest sesamoid bone in the body, and its articular cartilage is the thickest. Its primary function is to serve as a mechanical pulley for the quadriceps as it exerts an extension force across the knee. Its contribution increases with progressive knee extension and is most critical in the last 30° of extension. At full knee extension, the patella provides 31% of total knee extension torque, whereas between 90° and 120° of flexion it provides only 13% of torque.2 Because patellar instability occurs more commonly in the terminal 30° of knee extension, its effect could be significantly debilitating for the patient.
TYPES OF PATELLAR INSTABILITY BASED ON DIRECTION
Lateral patellar instability is the commonest form of patellar instability and would be further characterized later in this chapter. Patellar instability, by default, would mean lateral patellar instability.
Medial patellar instability is rare and disabling and almost always iatrogenic after lateral retinacular release.3 This form of instability and its treatment is described in Chapter 11. The presence of excessive femoral anteversion would present as inward (medial) position of the patella; this should not be misdiagnosed (or treated) as medial patellar instability (Figure 3.1).
Figure 3.1 A, B, Anteroposterior and lateral radiographs of 15-year-old girl misdiagnosed with idiopathic medial patellar dislocation and treated with lateral patellofemoral ligament reconstruction, which failed to relieve her symptoms. C, Full-length radiographs showing significant internal rotation of the femur with the feet facing forward. Computed tomography scan rotational profile revealed bilateral 40° femoral anteversion with internally rotated patella. D, Bilateral derotational osteotomy of femur relieved her symptoms.
Figure 3.2 Schematic diagram of superior (dotted arrow) and inferior (solid arrow) dislocation of patella.
Superior and inferior patellar dislocations have been described. Osteophytes or the lower end of the patella can lock against the proximal aspect of the trochlea (superior dislocation), or the superior end of the patella can lock against the intercondylar notch (inferior dislocation). The articular surface of the patella, in these dislocations, would face the trochlea (Figure 3.2).
Intra-articular patellar dislocation is a rotational dislocation around either the vertical or the horizontal axis. For vertical dislocation, the patella would spin medially or laterally, around a longitudinal axis, such that the articular surface would face outward. For horizontal dislocation, the patella would spin upward or downward, around a transverse axis, such that the articular surface would face either upward (proximally) or downward (distally). There may or may not be disruption of the extensor mechanism (Figure 3.3). In such complex patellar dislocation patterns, the patellar tendon could be traced proximally to help with correct orientation of the patella.
Figure 3.3 A, Vertical rotational dislocation of patella as seen on initial radiograph. B, After reduction, magnetic resonance imaging shows detachment of the quadriceps mechanism from the dorsum of the patella that allowed the patella to rotate and dislocate laterally, such that the articular surface faced outward. C, At surgery, the bare dorsal surface of the patella with peeling of the quadriceps mechanism was seen. Medial patellofemoral ligament reconstruction with reattachment of the quadriceps mechanism restored stability.
Multidirectional instability is very uncommon but could be seen in patients with severe ligamentous laxity or severe trochlear dysplasia.4 The patella could dislocate medially and laterally. Stabilization of such instability patterns would require both medial- and lateral-based reconstruction (Figure 3.4).
DEFINITIONS
Patellar dislocation is defined as movement of the patella out of the trochlea such that there is no articular contact between them. In the literature, the criteria used to identify patients with “true” patellar dislocation include (1) a dislocated patella requiring reduction or (2) a convincing history for dislocation, associated with full giving way episode, knee effusion, tenderness on the medial retinaculum, and positive apprehension sign.5 Other indirect signs suggesting lateral patellar dislocation include an avulsion fracture from the medial aspect of the patella on an axial view or a characteristic bone bruise pattern involving the medial aspect of the patella and the lateral aspect of the lateral femoral condyle on magnetic resonance imaging (MRI).6
Patellar subluxation is defined as a partial movement of the patella out of the trochlea but the presence of some articular contact between them. It is a confusing term that has been used interchangeably to reflect several different entities. It could be either a
radiographic/MRI finding, a subjective symptom of partial giving way/slipping of the patella out of the groove, an objective sign of increased displacement of the patella compared to the contralateral side, or a positive apprehension sign.7,8 Thus, whenever the term “subluxation” is used, its meaning has to be clarified.
Figure 3.4 A 15-year-old female with multidirectional patellar instability who was treated with medial and lateral patellofemoral ligament reconstruction.
Patellar instability encompasses patellar dislocation and patellar subluxation.
Patellar maltracking is defined as abnormal deviation of the patella from its normal tracking during knee flexion-extension. Attempts have been made to define normal patellar tracking, but there are variations. Amis et al defined normal tracking as about 4 mm of medial translation of patella as the knee is flexed from 0° to 20°; the patella thereafter would engage into the trochlea, and there would be a constant increase in lateral translation to about 7 mm at 90° of flexion.9 This path of engagement corresponds to the distal-lateral axis of the trochlear groove. An example of patellar maltracking is a “J-sign” where the patella slips out laterally during terminal knee extension, resembling an inverted J-path, and jumps back medially into the trochlear groove when the knee is slightly flexed from an extended position (Video 3.1
). The term “patellar maltracking” is not used routinely because of the ambiguity of its meaning, as mentioned later.
NOMENCLATURE
Grelsamer7 reported on several terms used to describe pathologies related to the patellofemoral joint and that lack proper definition. The use of terms such as chondromalacia, subluxation, maltracking, malalignment, realignment, patellofemoral syndrome, and anterior knee pain was discouraged unless precisely defined because they mean different things to different readers, often depending on the context.
The literature related to patellar instability could also be confusing because of the lack of standard terminology to describe various patterns of instability. For example, the term “mild” patellofemoral instability has been used in the literature, although no formal definition of “mild” or grade of severity of instability has been defined.10 Similarly, the term “chronic” patellar instability may either denote the time since the onset of patellar instability or may mean recurrent patellar instability.
In the literature, different types of patellar stability have been merged together, which can provide misleading information. For example, a study focused on habitual patellar dislocation included patients with recurrent patellar dislocation leading to misleading treatment recommendations.11
It is controversial as to what differentiates a J-sign from habitual dislocation in extension. They probably represent two ends of the same spectrum of instability, where the patella dislocates laterally every time the knee is extended from a flexed position. Chotel et al12 consider J-sign to be a mild form of habitual dislocation in extension. It can be defined as a gradual lateral “slide” of the patella in the terminal 30° of knee extension (Video 3.1). In contrast, habitual dislocation in extension would be at a higher (>30°) knee flexion, where the patella “jumps” out laterally as the knee is extended from a flexed position and then tracks laterally during further extension (bayonet sign) (Video 3.2
). The more severe forms are represented by patellar dislocation near full flexion that remains dislocated as the knee is extended.
Table 3.1 includes the definition of terms used to describe various forms of instability in this chapter.
CLASSIFICATION OF LATERAL PATELLAR INSTABILITY
Age and instability: Different instability patterns manifest at different ages.
Patellar instability in children under 10 years old is less common and represents some of the more severe or complex patterns of instability. If these childhood patterns are missed or neglected, then patients may present at an older age.
The adolescent age group has the highest incidence of patellar instability. This could be secondary to rapid growth spurts, hormonal influences during puberty, increased activity levels, and changes in the skeleton because of progressive ossification and growth. After this peak during adolescence, the incidence of patellar instability drops about 8% with each year of increase in age.13
In adults, arthritis may coexist with instability.
Various classification systems have been proposed for lateral patellar instability, based on clinical and imaging characteristics. No single system is considered the gold standard. We summarize the existing classification systems as follows.
TABLE 3.1 Nomenclature for Lateral Patellar Instability | ||||||||||||||||||
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Dejour et al
Dejour et al14 proposed a classification system of patellofemoral disorders based on pain or instability or both.
The classification was based on extensive radiographic study that identified four major anatomic abnormalities, including trochlear dysplasia, patella alta (Caton-Deschamps index > 1.2), patellar tilt (>20°), and increased tibial tubercle-trochlear groove (TT-TG) distance (>20 mm). The three patterns described were as follows:
Objective patellar instability (at least one documented dislocation and at least one anatomic abnormality; the term “objective” meant “obvious”)
Potential patellar instability (no dislocation but patellar pain in combination with anatomic abnormalities)
Patellofemoral pain (no dislocation or anatomic abnormalities, but patellar pain)
These patterns do not include other types of instability such as habitual or permanent patellar dislocation.
Garin et al
On the basis of a study of 50 knees with patellar instability in children and adolescents (average age 11 years, range 5-15 years), Garin et al15 proposed a classification system that distinguished patellar dislocation as being major or recurrent.
The major dislocation was subclassified as either permanent dislocation or habitual dislocation in flexion.
Recurrent dislocation was subclassified as either objective dislocation or potential dislocation.
Chotel et al
Chotel et al12 did not recognize potential patellar instability, as described by Dejour and Garin, and considered it a vague and controversial condition.
The authors proposed a more detailed classification system that distinguished five clinical patterns, more commonly seen in children. Their classification system did not include traumatic dislocation.
The clinical patterns of their classification system, based on the age of presentation, were as follows:
Congenital dislocation—occurred prenatally and presented at birth as irreducible fixed knee flexion contracture with other associated deformities permanent
Dislocation—developmental in nature and appeared once child started walking but before the age of 5 years
Habitual dislocation with knee flexion—appeared between the ages of 5 and 8 years; habitual dislocation with knee extension
Occasional, episodic, or recurrent dislocation— similar to objective patellar instability and appeared during preadolescence or adolescence. This was the most common and mildest form of instability.
Sillanpaa
Sillanpaa16 preferred to use the term “primary patellar dislocation” when patellar dislocation occurred for the first time and “secondary patellar dislocation” for recurrent dislocation.
The use of the term “acute patellar dislocation” was discouraged because it did not differentiate between a first-time dislocation and a recurrent dislocation.
Hiemstra et al
Hiemstra et al17 based their classification system on the shoulder instability categories of TUBS (traumatic, unilateral, Bankart lesion, surgery) and AMBRI (atraumatic, multidirectional, bilateral, rehabilitation, inferior shift).
Thirty-one patients, age greater than 14 years and with documented patellar instability, were separated into two main groups based on a scoring system—WARPS (weak, atraumatic, risky anatomy, pain, subluxation) or STAID (strong, traumatic, anatomy normal, instability, dislocation).
Patients were categorized into the WARPS group if they had diminished quadriceps muscle strength, core strength, and neuromuscular control; atraumatic or minimal traumatic onset of patellofemoral instability; several anatomic risk factors; and more pain or subluxation rather than frank dislocation.
Patients who were categorized into the STAID group had strong quadriceps muscles, traumatic onset of patellar dislocation of the patella, normal patellofemoral anatomy, and frank dislocation episodes.
Of 31 patients, 11 patients were in the WARPS subset, 16 were in the STAID subset, and 4 had mixed characteristics. This classification system is the only one that has undergone validity and reliability studies by the authors. However, it does not encompass instability patterns seen in children.
Frosch and Schmeling
In 2015, Frosch and Schmeling18 recommended a classification system based on instability criteria as well as on clinical and radiologic analyses of maltracking and loss of patellar tracking. They reported five types of patellar instability and maltracking patterns, along with management recommendations (Figure 3.5):
Type I was traumatic patellar dislocation without maltracking and instability, with a low risk of redislocation. It was rare and treatment was conservative.
Type II had a high risk of redislocation after primary dislocation but no maltracking. Medial patellofemoral ligament (MPFL) reconstruction would be the treatment of choice.
Type III represented both instability and maltracking. Maltracking was caused by (a) soft-tissue contracture or muscle imbalance, (b) patella alta, (c) pathologic TT-TG distance, (d) valgus deviation, or (e) torsional deformities. Isolated MPFL reconstruction may not suffice to treat patellar instability in the presence of maltracking. Additional osseous corrective surgeries may be required to achieve physiologic patellar tracking and to prevent redislocation.
Type IV featured a highly unstable “floating patella” with complete loss of tracking caused by severe trochlear dysplasia. Treatment of choice was trochleoplasty, combined with other bony and soft-tissue procedures.
Type V represented patellar maltracking without instability; for example, an internally rotated patella secondary to torsional deformity of the lower limb but without frank patellar dislocation. This would require corrective derotational osteotomy of femur and/or tibia.Related posts:
Surgical Considerations During Patellar Stabilization
Surgical Considerations for Tibial Tubercle Osteotomy
Lateral Retinacular Release and Lengthening
Chondral Lesions and Patellar Instability
Proximal Femur Rotational Osteotomy
Failed Medial Patellofemoral Ligament Reconstruction: Causes and Treatment
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